Cables, which are used for high-voltage electronic devices such as a medical CT apparatus and an X-ray machine and to which a high direct-current voltage is applied, are required to have (i) a small outside diameter and light weight, (ii) good flexibility and resistance against movement and bending, (iii) small electrostatic capacitance and followability to the repeated application of a high voltage, and (iv) heat resistance to resist against heat generation of an X-ray tube portion.
Conventionally, such a known cable for a high-voltage electronic device (e.g., a cable for X-ray machine) is formed by stranding two lines of low-voltage cable cores and one to two lines of bare conductors, forming an inner semiconducting layer on the strand, and sequentially forming thereon a high-voltage insulator, an outer semiconducting layer, a shielding layer and a sheath. For the high-voltage insulator, a composition based on an EP rubber (ethylene-propylene rubber) which is lightweight and flexible and has relatively good electrical characteristics is used (see for example, Reference 1).
In recent years, the EP rubber composition having a low dielectric constant (about 2.3) has been put into practical use, and it is being used as a material for a high-voltage insulator to develop a cable for a high-voltage electronic device having a smaller diameter (e.g., 75 kV class cable having an outside diameter of about 14 mm) and low electrostatic capacitance.
But, such a cable provided with a small diameter has a problem that its voltage resistance characteristic lowers because the high-voltage insulator becomes thin.